Cable Damage curves vs coordination

[julesm]

I'm trying to coordinate breakers using SKM software. I don't understand how the cable damage curves come in. Aren't cables damaged in more of an long-term heating "overload" situation and not in the .1 or 1.0 second range? I actually have not considered cable damage before so I would love some insight. I could coordinate my bldg beautifully if I ignored the damage curves!

 

[dpc]

jmadore,

The cable damage curve represents the maximum current that can flow through a cable for a given time without permanent damage to the insulation. It's a function of the insulation type and conductor size.

It's normally not a limiting factor in most coordination applications, since the fault should be cleared well before the damage curve is reached.

 

[davidbeach]

Cable damage is a consideration for through faults, is the protective device at the end of the circuit below the cable damage line? If so you don't need to worry about where the source protective device shows up.

With parallel cable runs use the N-1 option for the cable damage line. The N-1 line lets you determine if one faulted cable will damage the others as they feed the fault from the far end. The "1" line is way too restrictive when you have more than 2 parallel conductors and the "N" line doesn't look at faults in the parallel set.

Unless you have short, small circuits with high fault currents you shouldn't have any trouble with cable damage.

 

[EddyWirbelstrom]

PTW uses the adabiatic method ( no heat loss to environment )to determine conductor temperature in the event of a short-circuit.
I²t = K² S²
where :-
I = short-circuit current
t = short-circuit duration
K = constant depending on conductor specific heat, conductor continuous (initial) and short-time (final) permissible temp. eg for XLPE 90°C and 250° respectively.
Cable manufacturers and Standards will give values for K.
S = cable cross sectional area in mm²
When in the SKM-PTW library go General and click circular mils button. If the circular mils are not entered, the cable damage curve will not be draw. Even when PTW is configured for IEC standards and metric cables are being used, the cable libraries must also include the cable area in circular mils.
1 circ mill = area of circle 1/1000 inch diameter
The cable damage curve is drawn to ensure the cable is short-circuit protected.

 

[pauliwood]

Your breakers Time Current Curves (TCC) when overlayed on your cable damages curves, should appear to the left of your cable damage curves. Below 0.01s is the short circuit time frame, usually not shown on breakers as they are typically slower to open under fault conditions.

Not as familiar with breakers as I am fuses, yet, the curves work in the same manner for protection of your cables and equipment.

 

[EddyWirbelstrom]

The tcc time axis in SKM-PTW can be adjusted as follows :-
- open a tcc
- right click on a blank area of the tcc
- left click on TCC Settings...
- set the desired time axis ( and current axis ) ranges.
- click OK

 

[davidbeach]

What mnewman says is correct, but in my experience with previous versions of SKM it is rather worthless. The plots just don't work right (or didn't the last time I tried - no longer have access to SKM) unless the minimum time is 0.01s.

 

[EddyWirbelstrom]

Hello David, With the latest PTW ver 6034, the tcc curves plot below 0.01 sec if the library model has data for these values.

 

[alehman]

They work fine for me. I nearly always plot them as reference. In low-voltage systems with overcurrent protection selected per the NEC, damage is rarely a factor. It does come in to play where there are parallel conductors and you wish to protect the individual conductors.